1. Technical Field
The present disclosure relates to a sound reproduction device using narrow-directivity loudspeakers utilizing a parametric effect.
2. Description of the Related Art
FIG. 12 is a block diagram of conventional sound reproduction device 130, in which loudspeaker 111 that is a wide-directivity loudspeaker and superdirective loudspeaker 113 that is a narrow-directivity loudspeaker are placed in parallel with each other. Loudspeaker 111 is configured such that a sound pressure of audible sound from loudspeaker 111 decreases as distance d along a sound axis increases. When superdirective loudspeaker 113 faces a listener, a sound pressure of audible sound from superdirective loudspeaker 113 is maximized at predetermined distance dk from superdirective loudspeaker 113 along the sound axis. A position at which the sound pressure of the audible sound is maximized (the position of the maximum sound pressure) corresponds to listening point 126. Sound field 123 of the audible sound from loudspeaker 111 and sound field 125 of the audible sound from superdirective loudspeaker 113 overlap with each other at listening point 126. Here, superdirective loudspeaker 113 uses an ultrasonic wave as a carrier wave.
Loudspeaker 111 is electrically connected to sound source 119 such as a television set tuner, a CD player, and a DVD player via amplifier circuit 117. Superdirective loudspeaker 113 is electrically connected to sound source 119 via drive circuit 121.
Next, a sound pressure characteristic of sound reproduction device 130 will be described. FIG. 13A is a sound pressure characteristic diagram of audible sound of conventional sound reproduction device 130, with respect to distance d along the sound axis. FIG. 13A shows a relation of sound pressures of the audible sound from loudspeaker 111 and superdirective loudspeaker 113 with respect to distance d (along the sound axis) between a position at which loudspeaker 111 and superdirective loudspeaker 113 are placed and any point between the placement position and listening point 126. A horizontal axis (distance d along the sound axis) in FIG. 13A corresponds to a portion indicated by line Y-Y in FIG. 12. Further, a vertical axis in FIG. 13A shows sound pressures that are normalized respectively taking a maximum sound pressure of the audible sound from loudspeaker 111 and a maximum sound pressure of the audible sound from superdirective loudspeaker 113 as 1. The sound pressure of the audible sound from loudspeaker 111 is indicated by a dotted line, the sound pressure of the audible sound from superdirective loudspeaker 113 is indicated by a dashed line, and a combined sound pressure is indicated by a solid line.
Referring to FIG. 13A, the sound pressure of the audible sound from loudspeaker 111 is maximized at the position at which loudspeaker 111 is placed and decays as distance d along the sound axis increases. On the other hand, the sound pressure of the audible sound from superdirective loudspeaker 113 is small at the position at which superdirective loudspeaker 113 is placed, increases as distance d along the sound axis increases, is maximized at predetermined distance dk, and then decreases as distance d further increases. A sound pressure of sound superimposing the audible sound from loudspeaker 111 and superdirective loudspeaker 113 (combined sound pressure) is indicated by the solid line in FIG. 13A.
As shown in FIG. 13A, the sound pressures of the audible sound emitted from loudspeaker 111 and superdirective loudspeaker 113 are heard largest when the listener is positioned at predetermined distance dk along the sound axis from the position at which these loudspeakers are placed, and becomes smaller if the listener is away from predetermined distance dk.
Now, FIG. 13B shows a sound pressure characteristic of the audible sound with respect to distance w which is vertical to the sound axis (a portion indicated by line X-X in FIG. 12). Here, a vertical axis in FIG. 13B is the same as the vertical axis in FIG. 13A. The sound pressure of the audible sound from loudspeaker 111 is indicated by a dotted line, the sound pressure of the audible sound from superdirective loudspeaker 113 is indicated by a dashed line, and a combined sound pressure is indicated by a solid line. The sound pressure of loudspeaker 111 is maximized on the sound axis, and gradually decreases as distance w in a direction vertical to the sound axis increases. By contrast, the sound emitted from superdirective loudspeaker 113 has high directionality. Therefore, the sound pressure of the sound emitted from superdirective loudspeaker 113 is maximized on the sound axis. Further, in the direction vertical to the sound axis, the sound pressure of the sound emitted from superdirective loudspeaker 113 drops steeply as distance w increases. Thus, the sound pressures of the audible sound from loudspeaker 111 and from superdirective loudspeaker 113 (combined sound pressure) show a characteristic as shown by a solid line in FIG. 13B.
Combined sound pressure characteristics of the audible sound shown in FIG. 13A and FIG. 13B are shown in FIG. 14. As illustrated in FIG. 14, the peaks of the sound pressures correspond to listening point 126 both along the sound axis and in the direction vertical to the sound axis.
Further, a sound field realized by the conventional sound reproduction device is a sound field where the sound field of loudspeaker 111 overlaps with the sound field of the audible sound from superdirective loudspeaker 113 that reproduces the audible sound using an ultrasonic wave as a carrier wave. Therefore, a proportion of interference between the audible sound in the sound fields of loudspeaker 111 and of superdirective loudspeaker 113 is reduced as compared to that between sound fields produced from the conventional loudspeakers. Consequently, the listener is able to listen to the sound from superdirective loudspeaker 113 clearly, without being influenced by the sound from loudspeaker 111.
As described above, it is possible to realize a three-dimensional sound field that allows the listener to obtain a feeling that the listener is surrounded by sound, only with loudspeaker 111 and superdirective loudspeaker 113 that are placed in the same direction with respect to the listener, without providing a large number of loudspeakers around the listener.
As an example of the sound reproduction device using a wide-directivity loudspeaker and a narrow-directivity loudspeaker, International Publication No. WO2012/032704 is known.